The present invention relates to a filter for filtration of particles from a hot gas flow. The filter comprises a filter chamber, which is provided with a plurality of filter elements and at least one support and/or tube plate. The support and/or tube plate has apertures for the filter elements and it divides the filter chamber into at least two separate sections. The invention is especially suitable for filters in which filter tubes serve as filter elements.
Combustion and gasification methods presently used require reliable particle separators, which are capable of efficiently separating particles from hot flue gases or combustion gases. Especially, the hot gases produced in combustion or gasification taking place in a circulating fluidized bed reactor contain large quantities of particles, ash, bed material, unburned fuel, absorbents etc. In these conditions, the particle separator has to endure both heat and abrasion.
In modern, combined gas and steam turbine power plants, where gases are expanded in a gas turbine, efficient gas purification prior to the turbine is of crucial importance for steady operation of the turbine. Particles contained in hot gas considerably add to the corrosion and wear of the turbine vanes.
Today, environmental aspects also must be taken into consideration, i.e. to minimize the emissions of particles in all processes emitting gas into the environment.
Various particle separators, such as vertical cyclones, horizontal cyclones and different filters are used for purification of hot gases. During the last few years, heat resistant filters permeable to gases have proved to be good alternatives to conventional hot gas cyclones. Cyclones require space for both the cyclone itself and the support arrangements thereof, whereas filters are simple and require little space. Filters manufactured from porous special metals or ceramic material endure very high temperatures and, unlike cyclones, they do not require cooling or refractory heat insulations. In pressurized processes, the filters are readily connected to pressurized combustion or gasification systems.
In filters, particles gradually accumulate on the surface of the porous material and, consequently, the filter has to be cleaned every now and then. As a large amount of particles accumulates on the filter surface, a great pressure difference is needed between both sides of the filter for leading the gas therethrough. The filters may be cleaned by shaking if the filter material stands such treatment or, for example, by back-blowing clean gas through the filter so that the particles accumulated on the filter will be disengaged.
The great pressure difference between both sides of the filter, needed for leading the gas therethrough, results in that gas tends to pass from the side of the higher pressure, i.e., the inlet side for dirty gas, to the side of the lower pressure, i.e. the outlet side for clean gas, also via routes other than through the filter, for example, through the slots between the filter tube and the support plate. During the back-blow, i.e., the purification stage, the gas pressure is correspondingly raised momentarily higher on the clean gas side. It is important that the purification gas really passes through the filter and cleans it, not leaking through any other places that are not tight.
In a vertical filter, the filter tubes are normally attached at their upper ends, for example, via flanged joints to a horizontal support plate. The horizontal support plate also divides the filter chamber into two sections, one for dirty and the other for clean gas. The support plate has apertures through which the filter tubes run and to which they are attached. The slot between the filter tube and the aperture is sealed as tightly as possible so as to prevent dirty gas from escaping into a space for clean gas.
The slot between the filter tube and the support plate is designed to be sealed, e.g., by clamping the tubes tightly to the support plates. However, sealing is often very difficult because there are a great number of adjacent tubes disposed close together in the hot gas space, not leaving much space for clamping means and also making the filter structure very complicated.
It has also been attempted to seal the slot between the tubes and the apertures, for example, so that the aperture in the support plate tapers downwardly and the upper end of the filter tube is expanded upwardly in a curved manner, the tapered aperture thereby supporting the filter tube at the upper end thereof, at the same time sealing the slot between the tube and the aperture. German patent specification 35 15 365 discloses a gas filter of this type.
FI patent application 892368 suggests that the apertures in the support plate may be equipped with an inwardly directed flange, and the upper end of the filter tube correspondingly with an outwardly directed shoulder, and the space between the flange and the shoulder with a seal for sealing the slot between the tube walls and the support plate. The filter tube may be clamped by a press plate or a spring against the support plate. The filter tube may also be pressed against the support plate by placing a sufficiently heavy steel weight on the tube, such weight holding the tube tightly against the support plate. Use of weights remarkably adds to the load on the support plate.
In the above described known arrangements, the upper end of the filter tube has to be so shaped as to make its diameter larger than the diameter of the aperture in a corresponding plate, which places certain restrictions on the use of such arrangement. Vertical filters are often tall, which means that, besides the topmost support plate, the filter tubes usually require two or three additional support plates or possibly, for other reasons, the filter chamber has to be divided by tube plates into a plurality of spaces for clean gas. The slots between the apertures of these additional plates and the filter tubes also must be sealed.
It is much more difficult to install filter tubes if the filter is provided with several tube plates, especially, if the diameter of the filter tubes is not the same along the whole length thereof, but the tubes have shoulders for supporting said tubes or for sealing them with the tube plates. If the apertures for the tubes are equal in diameter in all support and tube plates, the tubes with shoulders have to be installed in the tube plates from the side, each tube from a level of its own. In this case, much installation room is required for transferring long filter tubes and for threading them into their respective tube plates.
It is possible--though hardly advantageous--to use filter tubes of different diameters on different levels, and correspondingly apertures of different diameters in different support or tube plates so that the tubes, including their shoulders, to be disposed on lower levels go through the upper apertures without being caught by the edges of the apertures until they have reached the correct level. In this way, all filter tubes to be disposed in the lower section go through the apertures in the upper plates until they reach their own level and will be stopped by the shoulders in the smaller apertures. In this manner, the filter area is, however, unnecessarily reduced in the lower section of the filter, because the filter tubes with smaller diameters also have a smaller filter area. Furthermore, idle space remains between the filter tubes in the lower section of the filter because the spacing of tubes is the same on both the topmost and the lowermost levels.
Fixed, tight joints between the filter tubes and the support or tube plates cannot be used in hot conditions due to unequal heat expansion of tubes and plates. The joint between the tubes and the plates has to be able to expand and contract. Different arrangements have therefore been suggested in which each tube is connected to the support or tube plate by means of bellows. However, the bellows easily wear, especially in hot conditions. U.S. Pat. No. 4,838,581 teaches a complicated arrangement in which an insulating layer is disposed between a hot filter tube and the bellows in order to protect the bellows from heat.
The present invention seeks to provide an improved filter, in which the drawbacks of the prior art have been minimized and, especially, a filter structure with simple and reliable sealing between the filter tube and the edge wall means defining the apertures in the support plate so as to prevent gas from flowing through the apertures. The invention enables use of seals made from simple, heat resistant materials, and the invention provides a joint between each filter tube and its support or tube plate, which enables movement of the tube caused by heat expansion, or the like. The invention also enables simple installation of the filter tubes and minimizes the risk of damage to the filter tubes during installation, and provides a filter structure in which all filter tubes may be installed from the same level. To achieve the above advantages, it is a desirable feature of the invention that the slot between the apertures in the support and/or tube plate and the filter elements extending therethrough is sealed by inserting at least two seals at a short distance from each other in the slot, the seals having substantially the same shape as the slot. The invention relates, e.g., to a filter with a plurality of adjacent filter tubes attached to circular apertures in the support plate, thereby forming annular slots between the aperture and the tube, the slots being sealed mainly by seals which are circular in shape. The seals are preferably made of elements in the shape of rings (e.g. a piston ring), each element being provided with a slot to give flexibility to the elements. Flexible elements may simply be shrunk, e.g., around the filter tube or may be clamped to the circumference of the aperture in the support or tube plate.
In order to attach the seal to the filter tube or to the aperture, it is preferable to make a groove in the tube or the surface of the aperture and to partly insert the seal in the groove.
The groove may be milled, e.g., directly in the surface of a ceramic filter tube or in the surface of the aperture in the plate. Technically, it may be more advantageous to fix a sleeve at the end of the tube or in the aperture in the plate and to make the grooves in the sleeve. The arrangement with sleeves may in some cases provide a better sealing result. A sleeve attached to the upper end of the tube may be provided with a flange, by means of which the tube is attached to the uppermost support plate.
The filter tubes may be so-called flow-through tubes, open at both ends thereof. Dirty gas is introduced into each filter tube from the upper end thereof. Clean gas passes through the filter formed by the tube wall and the particles separated from the gas partly stick to the filter, and partly flow downwardly inside the tube and out of the filter. The flow-through tubes may be supported from either their upper or their lower ends. Preferably, the tubes are attached to the uppermost support plate by means of a flanged joint or the like.
The filters may also be closed at their lower ends, whereby the dirty gas is introduced into a gas space outside the filter tubes, and the clean gas passes through the filter surface into the tubes and further out of the filter. The particles remain on the outer surface of the filter tubes and come off the surface by themselves or by means of, back-blowing, and then may be removed from the filter.
The efficiency of the seal between the tubes and the plate may be improved by leading sealing or extrusion gas, for example, air or other clean gas, into the slot between the tube and the plate. The extrusion gas fed into the slot prevents, by means of overpressure formed thereby, dirty gas from flowing from a volume of dirty gas through the slot into the volume of clean gas. The same gas, e.g. air, as is used for cooling the support or tube plates may serve as a sealing gas. The plate may be simply provided with gas feed openings close to the slot to be sealed, through which openings gas continuously flows into the slot.
In accordance with an arrangement according to the invention, gas may be fed into grooves formed in the surface of the apertures in the plate, e.g. to the backside of the sealing ring so that the gas fed thereinto presses the sealing against the wall of the filter tube thereby increasing the sealing efficiency.
The sealing arrangement according to the invention enables use of filter tubes which are substantially of the same thickness throughout the whole length thereof without causing any deterioration of the sealing efficiency. Filter tubes of the same thickness throughout the whole length thereof are easy to install because all tubes may be brought into the filter through one installation opening, for example, in the upper or lower section of the filter. Therefore, separate, large installation openings for the tubes are unnecessary at the filter sides.
The sealing may be formed simply by using two or more simple sealing rings. Neither a bellows nor other complicated, space consuming, heavy and expensive seals are needed. For example, seals in the shape of a piston ring are simple to clamp on the tubes or in the apertures.
The sealing according to the invention enables movement of filter tubes caused by heat expansion in the aperture of the support plate, and is light, thereby not significantly adding to the load on the support plate.